1. Field of the Invention
The present invention relates to a hydraulic damper for use on a motor vehicle or the like, and more particularly to a damping force adjusting mechanism for such a hydraulic damper.
2. Description of the Prior Art
Japanese utility model publication No. 53-26550 discloses a hydraulic damper for use on a motor vehicle. The disclosed hydraulic damper has a cylinder, a hollow rod extending into the cylinder, and a piston fixed to the inner end of the hollow rod and held in sliding contact with an inner surface of the cylinder. The piston has an oil passage for generating damping forces and a valve for selectively opening and closing the oil passage. A bypass oil passage is defined to bypass the piston through the hollow passage, and an adjusting rod is axially movably inserted in the hollow rod and disposed in the bypass oil passage. When the adjusting rod is axially moved in the hollow rod, the cross-sectional area of the bypass oil passage is varied to adjust the damping forces produced by the hydraulic damper.
The disclosed damping force adjusting mechanism cannot instantaneously adjust the damping forces in response to any of various sensor signals because an eccentric cam for axially moving the adjusting rod is manually turned.
Another known hydraulic damper disclosed in Japanese laid-open utility model publication No. 60-24688 is similar to the hydraulic damper disclosed in Japanese utility model publication No. 53-26550 except that a conical adjusting screw is used instead of the eccentric cam, and the adjusting rod is axially movable by an actuator through the adjusting screw. The adjusting rod has one end facing into an oil chamber in the cylinder which is pressurized by a gas under pressure. The adjusting rod is thus subject to a force applied by the gas under pressure and tending to axially move the adjusting screw to a position in which the generated damping forces are smallest. When the motor of the actuator or a control system for controlling the actuator fails and the actuator is into a free condition, the adjusting screw is axially moved into the position in which the damping forces are smallest, with the result that the damping forces become inappropriate. The damping forces also become inappropriate when an error signal is applied to the actuator due to a failure of the control system.